As a new type of solvent with immeasurable vapor pressure, room-temperature ionic liquids are being used for chemical separation and unique reaction media. Solvent phase behavior is an important factor in the attractiveness of using ionic liquids in these applications as well as in new applications such as absorption cooling or heating.
The absorption cooling and heating cycle is a more-than-100-year-old technique, and is well known from descriptions such as that by Haaf et al in “Refrigeration Technology” (Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, Wiley-VCH Verlag GmbH, Weinheim, Germany, Volume 31, pages 269-312). The basic cooling cycle uses a low-temperature liquid refrigerant that absorbs heat from water, air or any medium to be cooled, and converts to a vapor phase (in the evaporator section). The refrigerant vapors are then compressed to a higher pressure by a generator, converted back into a liquid by rejecting heat to the external surroundings (in the condenser section), and then expanded to a low-pressure mixture of liquid and vapor (in the expander section) that goes back to the evaporator section and the cycle is repeated. An absorption system uses heat for compressing refrigerant vapors to a high-pressure.
Although the vapor compression cycle is now used in most air-conditioning and refrigerating applications, the well-known refrigerant-absorber systems (H2O/LiBr and NH3/H2O) are still being used for certain applications, particularly in the field of industrial applications or large-scale water chiller systems. Recently, more attention has been directed toward recovery of waste heat using the NH3/H2O system (Erickson et al, Heat-Activated Dual-function Absorption Cycle, ASHRAE Trans., 2004, 110). Inherent drawbacks to using LiBr and NH3 as refrigerants include the corrosiveness of LiBr and the toxicity and flammability of NH3.
In the late 1950s, some pioneering works proposed new refrigerant-absorbent pairs for the absorption cycle, using fluoroalkane refrigerants with organic absorbents (Eiseman, A Comparison of Fluoroalkane Absorption Refrigerants, ASHRAE J. 1959, 1, 45; Mastrangelo, Solubility of Some Chlorofluorohydrocarbons in Tetraethylene Glycol Ether, ASHRAE J. 1959, 1, 64). Such studies continue actively even at the present time, especially among academic institutions [Nezu et al, “Thermodynamic Properties of Working-Fluid Pairs with R-134a for Absorption Refrigeration System”, Natural Working Fluids 2002, IIR Gustav Lorentzen Conf. 5th. China, (Sep. 17-20, 2002, 446-453); Fatouh et al, “Comparison of R-22 Absorption Pairs for Cooling Absorption Based on P-T-X Data”, Renewable Energy, 1993, 3, 31-37; Bhatt et al, “Thermodynamic Modeling of Absorption-Resorption Heating Cycles with Some New Working Pairs”, Heat Recovery System & CHP, 1992, 12, 225-233.]
Although U.S. patent application Ser. No. 11/346,028, which is incorporated in its entirety as a part hereof for all purposes, discloses an absorption cycle wherein refrigerant pairs comprising at least one refrigerant and at least one ionic liquid are utilized, a need remains for systems to run an absorption cycle utilizing a selected pairs of refrigerants and ionic liquids.